1. Field of the Invention
Embodiments of the present invention relate generally to fuel cells which utilize a current collector and/or a separator for the purpose of providing an electronic flow path for current generated by the fuel cell, to support the electrodes and electrolyte holding member, and to form the flow field for gas access to electrodes.
2. Description of Related Art
Conventional planar fuel cell stacks typically are comprised of a plurality of fuel cell sub-assemblies arranged in an electrical series relationship. Each fuel cell sub-assembly may be comprised of an anode electrode, a separator plate, and a cathode electrode. Each electrolyte holding member is located between adjacent fuel cell sub-assemblies so as to be in contact with the anode and the cathode of adjoining fuel cell sub-assemblies. Another approach is to provide a plurality of membrane-electrode assemblies, or MEA's, with the separators located between adjacent MEA's. At assembly, the fuel cell stack is compressed axially to afford good intimate contact at each interface of the fuel cell stack to establish the electronic flow path for the electrons liberated by the electrochemical fuel cell reaction.
The separator plate, being disposed between adjacent anodes and cathodes, is required to be constructed from a conductive material. Typically, the basis for selection of material to construct the separator is a function of the operating characteristics of the fuel cell type. Each of the various fuel cell types has its particular electrolyte and operating temperature and provides various degrees of operating efficiencies. Typically, fuel cells which operate at low temperatures (˜<400 C.) may utilize a polymer separated carbon graphite for the separator material. Fuel cells which operate at temperatures greater than ˜400 C. utilize stainless steels and ceramics as the separator material.
The separator plate of a conventional high temperature fuel cell stack serves multiple purposes.                The separator acts as a housing for the reactant gasses to avoid leakage to atmosphere and cross-contamination of the reactants.        The separator acts as a flow field for the reactant gasses to allow access to the reaction sites at the electrode/electrolyte interfaces.        The separator further acts as a current collector for the electronic flow path of the series connected fuel cells.        
In many cases the separator is comprised of multiple components to achieve these purposes. Typically, three to four separate components, or sheets of material, are needed depending upon the flow configuration of the fuel cell stack. It is frequently seen that one sheet of material is used to provide the separation of anode/cathode gasses while two additional sheets are used to provide the flow field and current collection duties for the anode and the cathode sides of the separator. Another example of prior art is to use one sheet of ribbed or dimpled material to create the anode/cathode separation as well as the flow fields. Additional sheets of perforated material are used for current collection and, in some instances, to enhance the flow field cross sectional area.
There are three fundamental flow patterns of the reactant gasses which may be applied to the separator to achieve varying objectives. The three patterns consist of co-flow, counter-flow, and cross-flow. Each of the three flow patterns introduces varying degrees of complexity to the design and construction of the separator and current collectors. Low temperature fuel cells, employing carbon graphite as the separator material, often will utilize a combination of the three fundamental flow patterns resulting in sinusoidal flow paths or “Z” patterns.
U.S. Pat. No. 4,548,876 teaches the application of a “corrugated metallic electron collector” which “includes a plurality of corrugations therein”. A preferred embodiment described in this patent utilizes particles within the metallic electron collector to “provide support for a respective catalyst (i.e. electrode) immediately adjacent to and in contact with the metallic electron collector”. These collectors are adjacent to flat “separators”.
This approach has been further advanced through the application of an additional sheet metal component comprised of a perforated sheet positioned between the “corrugated metallic electron collector” and the respective electrode. These approaches have proved to be technically feasible, however, the material content of such structures is economically prohibitive, consisting of three to five sheet metal components of rather significant complexity.
U.S. Pat. Nos. 4,654,195 and 5,531,956 among others teaches the application of “ribs” to the anode electrode of the fuel cell. This approach is intended to apply the flow field directly to the electrode. While technically effective, typically the material cost of the anode is greater than that of sheet metal used to otherwise form an anode flow field with the current collector and separator. Additionally, depth of the ribs formed on the anodes is insufficient for large area fuel cells requiring large cross-sectional flow area. Furthermore, excessive mechanical creep of ribbed anodes can result in poor performance of the fuel cell.
U.S. Pat. No. 4,983,472 teaches the application of a “plurality of arches” to the current collector in a somewhat similar fashion as the above mentioned U.S. Pat. No. 4,548,876. However, the plurality of arches are distributed much more densely and create a finer degree of support to the electrodes thus eliminating the requirement for supporting particles or an additional perforated sheet metal component. This approach has proven to be technically successful but yet again has not reduced the component count of the separator plate below three sheets of material, two current collectors and one separator sheet.
U.S. Pat. No. 5,503,945 teaches the application of corrugations to the “main plate” of the separator and the use of perforated current collector for both the anode and cathode. This patent further teaches the integration of the current collector with its respective electrode. Additionally, this patent teaches the integration of the current collector of either the anode or the cathode with the peripheral sealing structure of the separator and claims a two piece separator with reduced material content and component count. However, the requirement for a current collector for the anode and for the cathode have not been eliminated. The active central area of the fuel cell typically constitutes the far greater portion of the area of the fuel cell relative to the peripheral sealing area. Therefore, while component count of the separator assembly has been reduced through integration of one of the current collectors with either the anode or with the cathode, the material content and component count of the separator as a whole has not appreciably been altered when viewing the total assembly. Furthermore, current collector/separator designs which utilize a ribbed separator and a nominally flat perforated current collector suffer from diminished cross-communication of the reactant gas from one rib to adjoining ribs.
U.S. Pat. No. 5,795,665 teaches the application of “a plurality of rows of dimples” to the separator plate and to the “current collector/active component subassembly”. Though resulting in modest reduction of material content the separator/current collector component count remains at three. This invention provides for cross-flow, co-flow, or counter-flow of reactant gasses utilizing three sheets of material.
U.S. Pat. No. 5,811,202 teaches the application of ribs to an “anode field plate” and to a “cathode field plate” separated by a “flat middle plate”. A perforated current collector is disposed between the cathode field plate and the cathode electrode throughout the central active area of the fuel cell. Again, as with the above mentioned U.S. Pat. No. 5,503,945, the active central area of the fuel cell typically constitutes the far greater portion of the area of the fuel cell relative to the peripheral sealing area. Therefore, while component count of the separator assembly has been stated as being three, the component count of the separator as a whole when viewing the total assembly is four.
Material content of the separator/current collector assembly tends to be a function of two factors: the size of the active area of the fuel cell and the efficient use of structural forming materials. Fuel cells with areas exceeding about one square foot and operating at or near atmospheric pressures require reactant flow fields (i.e. anode and cathode) with sufficient cross-sectional area to prevent excessive pressure build-up at the inlets. Excessive pressure at the inlets relative to the outlets can create undesirable pressure differentials that may contribute to leakage of the reactants. As such, as cell area, and consequently flow field length, is increased to provide ever greater quantities of power output the cross-sectional area of the reactant flow fields also must increase. As it is the material of construction of the separator which forms the flow fields, material content of the separator rises with the increase in cross-sectional flow area. Excessive material content can be controlled with efficient use of the structural forming materials and the limitation of flow field length.
Material thickness of the various current collector designs is governed by several factors which include corrosion rates of the various fuel cell environments as well as the mechanical constraints induced by the axial compressive load applied to the fuel cell stack and the unsupported spans of the flow field induced by the desire to maximize the cross-sectional flow area. As mentioned above, as cell area increases the cross-sectional flow area also is increased to accommodate the added reactant gas flow rate and associated back pressure. As a result fuel cells which seek large areas and/or operate at atmospheric pressures must provide relatively large cross-sectional flow area which tends to manifest in wide unsupported spans requiring robust current collector design.
In some instances it has been found necessary to electroplate a stainless steel alloy with nickel for corrosion protection in anode current collector applications. Nickel is very stable in the anode environment of the molten carbonate fuel cell but does not retain the same degree of strength at operating temperatures as certain stainless steel alloys. When the current collector is configured to provide high degrees of structure to the flow field, as with U.S. Pat. No. 4,983,472, a high strength alloy may be utilized to produce the anode current collector and typically is nickel plated following the forming process. Nickel plating adds significant expense to the manufactured cost of the current collector.
All of the above described prior art utilizes perforated sheet metal in one form or another to create the current collector of the fuel cell. These perforations are arranged in highly repetitive patterns to simplify manufacture as well as to maximize the access of the reactant gasses to the electrodes. The degree of reactant access to the electrode is often referred to as “percent open area” of the current collector. A reasonably large percent open area is needed to avoid choking the fuel cell electrochemical reaction which diminishes performance. However, large percentage of open area alone neglects the electrodes requirement for physical support against the stack axial compressive load. A compromise is made to limit percentage of open area while maintaining appropriately dispersed electrode support.
For example, molten carbonate fuel cell electrodes can sustain only modest widths of open and unsupported area of approximately 0.10 inches. Additionally, the performance of molten carbonate fuel cells is diminished if the width of the areas of the current collector supporting the electrode exceeds approximately 0.10 inches. As a result the typical MCFC current collector pattern of perforated openings has a pitch in at least one axis of approximately 0.20 inches. These factors become more restrictive as electrode thickness is reduced.
However, it has been observed that the electrodes are able to sustain considerable lengths of narrow unsupported areas. For example, the pattern described in the above mentioned U.S. Pat. No. 4,983,472 patent where, although the width of the typical opening is held at 0.06″, the length of the opening is 0.190″. Another example is the description in the above mentioned U.S. Pat. No. 5,811,202 patent which implies that the anode is supported only by a ribbed flow field implying significant lengths of narrow unsupported area.
In some instances, as with the current collector described in patent U.S. Pat. No. 4,983,472, the perforations are partial and the otherwise scrap metal is utilized to form the structure of the cross-sectional flow area. As such, the manufacture of these various current collector designs typically requires specialized tooling operated in press machines of varying degrees of complexity. In current collector patterns where the perforation is total and a through hole is produced a large quantity of scrap metal is generated during the manufacturing process, up to 35-40%. The complexity of the tooling and the generation of large quantity of scrap conspire to render the various designs as uneconomical at high production volume anticipated for commercial fuel cell applications.
As mentioned, the typical highly repetitive patterns are intended to maximize gas access to electrodes for optimum fuel cell performance. However, the various flow patterns of the separator and the vagaries of the electrochemical fuel cell process often result in undesirable concentrations of electrochemical reactions. This tends to create hot spots and cold spots, or thermal gradients, in the fuel cell which can contribute to premature failure of the fuel cell. It is well established in the art that counter-flow of reactant gasses in general provides the optimum distribution of the electrochemical fuel cell process and results in good current density distribution and reduced thermal gradients. In such a counter-flow approach the fuel stream in each cell is initially directed to a region of the anode layer witch coincides with the region of the adjoining cathode layer in which the oxidant stream has the lowest concentration of oxygen. As the fuel stream progresses through the fuel cell and approaches the exit it has been depleted of fuel and now coincides with the region of the cathode in which the oxidant stream has the highest concentration of oxygen. It is further well established in the art that co-flow and cross-flow of reactant gasses provide for a less optimum current density distribution and thermal gradients. It is additionally well established in the art that the current collector of a fuel cell is capable of diminishing the performance of the fuel cell if the current collector shields excessive areas of the electrodes from the reactant gases.